The present invention relates to the field of calenders, and more particularly to devices for controlling the diameter of rolls used in calenders or analagous machines.
Pressing a material between two calender rolls can change the physical charateristics of the material. For example, calendering paper can change its density, thickness and surface features. Thus, the calendering process is frequently used in the manufacture of paper and other sheet materials where it is often desirable to change the density, thickness or surface features of the material.
A common problem associated with calendering is an uneven thickness of the sheet of calendered material. Localized variations in a variety of parameters affect the diameter of individual calender rolls and create variations in the spacing or "nip" between cooperating rolls. Variation in the nip across the width of a pair of calender rolls produces a sheet having non-uniform thickness. Thus, a more uniform thickness could be obtained if the local diameters of the calender rolls could be controlled.
If a calendar roll is made of a material that responds to changes in temperature, one may control local roll diameters by varying the temperature of selected cylindrical sections or "slices" of the roll. Previous devices use this principle by directing infrared heat radiation against the surface of slices of a rotating calender roll to control the local diameters of the roll. This infrared heating method, however, is inefficient since the absorptivity of the polished wrought iron surface of most calender rolls is very low, about 0.28. Therefore, instead of heating the calender roll, most of the infrared radiation directed against the roll is reflected. The present invention provides a more efficient means of utilizing infrared radiation to heat a calender roll.
Other types of calender roll control devices direct jets of hot or cold air against slices of a rotating calender roll to control its local diameters. Many of these devices blow hot air from a hot air plenum against slices of the calender roll to increase the local diameter of the roll and thus decrease the local thickness of the sheet of calendered material. Alternatively, when these devices release cold air from a cold air plenum against selected slices of the calender roll, those slices contract. This decreases the local roll diameter and increases the local thickness of the sheet of calendered material.
These air jet devices are subject to certain limitations and inefficiencies. For example, the nip control range is determined by the maximum and minimum temperatures of the air jets. The air in the hot air plenum is typically heated by waste steam from the facility power plant. However, waste steam supplied by the power plant generally has a maximum temperature of about 350.degree. F. and inefficiencies in the heat exchange process further limit the maximum temperature of steam heated air to about 325.degree. F. Examples of such devices are shown in U.S. Pat. No. 4,114,528 to Walker and U.S. Pat. No. 3,770,578 to Spurrell.
The calender roll control device of the present invention has a number of features which overcome many of the disadvantages of air jet control devices heretofore known. For example, the infrared heat lamps used by the present invention to heat the calender roll are capable of achieving higher temperatures than steam heated air. This higher temperature provides a greater nip control range. Additionally, the relatively low efficiency of heat transfer between the air jets and the calendar rolls results in a relatively slow response time and a limited ability to affect the roll diameters. The device of the present invention provides a more rapid and efficient means for heating the calender rolls with infrared radiation.
Another type of previously known calender roll control device uses magnetic fields to heat the calender roll. An example of this type of device is shown in U.S. Pat. No. 4,384,514 to Larive et al. In this type of device, the calendar roll is made of a conducting material and magnets are positioned close to the roll surface. As the rotating roll passes under the magnets, slices of the roll are heated by magnetic induction. The magnetic fields induce currents in the calender roll which dissipate their energy by heating the roll. However, because 50/60 Hz magnets have high magnetic forces which may bend the roll, 25 KHz alternating current electromagnets are generally used. Thus, workable magnetic induction calender roll control devices generally require a special alternating current power supply.
Furthermore, to achieve the greatest heating effect, the magnets generally should be positioned within about one-eight inch of the calender roll surface. However, placing the magnets this close to the calender roll may lead to damage when the sheet of calenderable material breaks. A broken sheet can wrap around the roll a sufficient number of times to build up a thick layer of calendered material on the roll. Once this layer becomes more than one-eight inch thick, the rotating calender roll can drive the material into the magnets with sufficient force to damage both the magnets and their supporting structure.
The device of the present invention also provides a number of advantages over magnetic induction calender roll control devices. For example, the infrared reflectors used in the present invention to direct infrared radiation from the infrared heat lamps toward the calender roll are generally positioned approximately two inches from the roll surface. This two inch between the reflectors and the calender roll greatly decreased the possibility of damage to the reflectors by contact with the calendered material. Additionally, the device of the present invention is generally less expensive and easier to service than magnetic induction devices since it does not require a special alternating current power supply.
The present invention thus provides a number of advantages over prior art calender roll control devices. These and other advantages will become apparent in the description which follows.